1. Field of the Invention
The present invention relates to a display device such as a liquid crystal display device or an EL display device, and more particularly to a technique for reducing wiring from a video line drive circuit or a scanning line drive circuit to a display panel.
2. Description of the Related Art
Currently, as a liquid crystal display panel which is used in a liquid crystal television receiver set, a mobile phone or the like, a TFT-type liquid crystal display device is used. FIG. 1 shows an equivalent circuit of a conventional TFT-type active matrix liquid crystal display panel.
As shown in FIG. 1, the conventional liquid crystal display panel includes, on a liquid-crystal-side surface of one substrate of a pair of substrates which is arranged to face each other in an opposed manner with liquid crystal sandwiched therebetween, a plurality of scanning lines (also referred to as “gate lines”) (GL) and a plurality of video lines (also referred to as “source lines” or “drain lines”) (DL).
Regions surrounded by the scanning lines and the video lines constitute sub pixel regions, and each sub pixel region is provided with a thin film transistor (TFT) which has a gate thereof connected to the scanning line, a drain (or a source) thereof connected to the video line, and the source (or the drain) thereof connected to a pixel electrode (PX) and constitutes an active element.
Since liquid crystal is interposed between the pixel electrode (PX) and a counter electrode (CT), a liquid crystal capacitance (Clc) is formed between the pixel electrode (PX) and the counter electrode (CT). Although a holding capacitance (Cadd) is provided between the pixel electrode (PX) and the counter electrode (also referred to as “common electrode”) (CT) in the actual constitution, the description of the holding capacitance (Cadd) is omitted in FIG. 1.
The respective scanning lines (GL) are connected to a vertical scanning circuit (also referred to as “gate driver”) (XDV), and the vertical scanning circuit (XDV) supplies a selection scanning signal to the respective scanning lines (GL) sequentially. The respective video lines (DL) are connected to a horizontal scanning circuit (also referred to as “source driver” or “drain driver”) (YDV), and the horizontal scanning circuit (YDV) outputs video voltages (so-called grayscale voltages) of R, G, B to the respective video lines (DL) within 1 horizontal scanning period. Here, JP-A-2007-140296 (patent document 1) discloses a technique which can decrease the number of outputs of a driver which drives video lines by performing the selection of the video lines by time division using RGB switches.
With respect to the thin film transistor (TFT), there has been known a thin film transistor in which a semiconductor layer is formed of an amorphous silicon layer (hereinafter referred to as “a-Si thin film transistor”), and a thin film transistor in which a semiconductor layer is formed of a polysilicon layer (hereinafter referred to as “poly-Si thin film transistor”). Recently, there has been also known a thin film transistor (TFT) in which a semiconductor layer is formed of a microcrystalline silicon layer (hereinafter referred to as “microcrystalline thin film transistor”). The microcrystalline thin film transistor exhibits performance between the performance of the a-Si thin film transistor and the performance of the poly-Si thin film transistor.
In general, a liquid crystal display panel for a liquid crystal television receiver set uses the a-Si thin film transistor as an active element thereof, and a liquid crystal display panel for a mobile phone uses the poly-Si thin film transistor as an active element thereof.
An operating speed of the poly-Si thin film transistor is two-orders of magnitude faster than an operating speed of the a-Si thin film transistor. Accordingly, in the liquid crystal display panel which uses the poly-Si thin film transistor as the active element thereof, the vertical scanning circuit (XDV) is formed of the poly-Si thin film transistor, and the vertical scanning circuit (XDV) is formed on a liquid-crystal-side surface of one substrate of a pair of substrates which constitutes the liquid crystal display panel.
The operating speed of the a-Si thin film transistor or the operating speed of the microcrystalline thin film transistor is slower than the operating speed of the p-Si thin film transistor. Accordingly, in the liquid crystal display panel which uses the a-Si thin film transistor or the microcrystalline thin film transistor as the active element thereof, a semiconductor chip which mounts the vertical scanning circuit (XDV) thereon is mounted on one substrate of a pair of substrates which constitutes the liquid crystal display panel, for example.
In general, as a method for mounting a semiconductor chip which constitutes the vertical scanning circuit (XDV) and the horizontal scanning circuit (YDV), there has been known a method in which a semiconductor chip which constitutes the vertical scanning circuit (XDV) and a semiconductor chip which constitutes the horizontal scanning circuit (YDV) are separately mounted on one substrate of a pair of substrates which is arranged to face each other in an opposed manner with liquid crystal sandwiched therebetween as shown in FIG. 1, and a method in which a semiconductor chip which constitutes a scanning circuit (RDV) which is formed by integrating the vertical scanning circuit (XDV) and the horizontal scanning circuit (YDV) is mounted on one substrate of a pair of substrates which is arranged to face each other in an opposed manner with liquid crystal sandwiched therebetween as shown in FIG. 2.
In either method, a selection scanning voltage is supplied to the respective scanning lines (GL) from the vertical scanning circuit (XDV) (or the scanning circuit (RDV)) and hence, it is necessary to provide the gate lines which connect the vertical scanning circuit (XDV) (or the scanning circuit (RDV)) and the respective scanning lines (GL) in such a manner that the number of gate lines is equal to the number of the scanning lines (GL).
However, in a miniaturized panel such as a liquid crystal display panel for a mobile phone or the like, when the number of pixel is increased to satisfy a demand for higher definition, there exists a possibility that gate lines cannot be arranged in the inside of the liquid crystal display panel. To cope with such a situation, considered is a line address drive method where in driving the scanning lines (GL), only scanning lines (GL) whose addresses are designated are selected.
In FIG. 1 and FIG. 2, symbol VSYNC indicates a vertical synchronizing signal, symbol HSYNC indicates a horizontal synchronizing signal, symbol CK indicates a dot clock, and symbol Data indicates video data.